Eli5: why do diesel engines tend to be turbocharged rather than supercharged or even N/A?
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Supercharging increases the maximum power of an engine but reduces efficiency. You lose a lot of energy simply in the act of increasing the amount, pressure, and temperature of, hot gas coming out of the exhaust which increases noise significantly. So you reduce the fuel economy. A decent fraction of the gross power you gain is consumed off the crankshaft to run the SC.
In most applications for diesel engines, reducing net efficiency and fuel economy isn’t desirable.
Turbocharging takes energy that would normally just fly out the tailpipe to compress incoming air. No parasitic load necessary (or not very much.) Since in diesels the increase in mass airflow and manifold pressure need not necessarily mean increasing the amount of fuel, typically the turbocharger increases efficiency by increasing total compression ratio. The opposite is sometimes true with gasoline engines but that depends on the engine parameters
Supercharging is sometimes used on two-stroke diesel engines, to increase the *scavenging* throughput at the bottom of the cycle when the ports are exposed. This increases torque at engine speeds lower or higher than the power band of the engine, and also makes starting easier. Typically they’re also turbocharged. Two stroke diesels are unusual but they’re most often used on large marine engines or engines for power generation. Their advantage is that they have potentially higher torque at low RPM in the case of driving a propeller directly without a reducing gearbox. The most well known example of a supercharged diesel for automotive were the Detroit Diesel series 71 of engines.
Supercharging used to be common in piston engines on high performance military aircraft. The main reason for this was to increase their performance at high altitude where the air pressure and density was low, starving the engines for air. Typically you could gain some efficiency that way by using a Miller cycle, ie holding the intake valves open slightly longer and reducing the length of the compression stroke.
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